1. Field of the Invention
The present technology relates to nonvolatile memory cells, and in particular to nonvolatile memory cells subject to the program disturb effect.
2. Description of Related Art
The program operation of a nonvolatile memory cell is complicated by the program disturb effect. Programming refers to adding charge to, or removing charge from, selected memory cells of a memory array, unlike the indiscriminate erase operation which resets typically an entire sector of memory cells to the same charge storage state. The invention encompasses both products and methods where programming refers to making the net charge stored in the charge trapping structure more negative or more positive, and products and methods where erasing refers to making the net charge stored in the charge trapping structure more negative or more positive. In the program disturb effect, programming of a selected cell leads to unwanted programming of unselected memory cells adjacent to the selected cell. In particular, the program disturb effect leads to unwanted programming of memory cells that are: 1) located in columns adjacent to the column including the selected cell and 2) connected to the selected row line (the word line providing a gate voltage to the selected cell). The integrity of the memory array is adversely affected by these problems For example, the drain-to-source voltage Vds of the disturbed cell can be about 2.5 V, and drain current about 35-45 uA, shifting the threshold voltage more than 50 mV.
A prior approach of addressing the program disturb effect attempts to alleviate the conditions giving rise to the unwanted programming of unselected memory cells, Unselected memory cells are programmed because of an unwanted voltage difference across the bit lines connected to the unselected memory cells which are in the columns nearby the column of the selected cell. For example, if a bit line voltage is raised to program a memory cell positioned on one side of the bit line, then the program disturb effect tends to program the adjacent memory cell on the other side of the bit line as well. Applying a program voltage to not just a bit line adjacent to the memory cell to be programmed, but also to additional bit lines of memory cells that would otherwise be subject to the program disturb effect, does not appear to solve the program disturb effect. These additional applications of the program voltage effect to other bit lines changes the location of the disturbed cell, instead of prevent the program disturb effect.
Therefore, it would be advantageous to address the program disturb effect, rather than simply shift the location of the program disturb effect.